Normally, I would just wipe and reinstall as "activators" are mostly virus-infected, but since you want to continue using your version, try this. I hope this helps, and, remember, don't forget to use a virus scanner for everything.

EDIT: On second thought, I'll post it here in case that link was deleted and for others to see.

Code:

Make Windows XP Genuine/ Remove Window Piracy Message
Follow the steps
1. Click on Start Button
2 Goto Run option or directly press Window key+R
3. Type notepad in Run Box, and press Enter Key or click on OK Button
4. Now paste the below text in red color in the notepad
5. Now save the notepad file with xp.reg using File->Save As option on your desktop or any other location.
6. Now close the notepad and open the saved file xp.reg from the location where you saved the file.
7. Restart your PC